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Caution - 14080_147
Screen Selection

Equipment Operator Advanced - Advanced construction equipmet operators manual
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setting may result in packing in the crusher chamber, high loads and stresses, and possible damage to major components. After passing the  dual roll crusher, the material is recirculated to the top screen by means of a return conveyer and a revolving elevator wheel. The material from the hydrocone crusher is recirculated to the top screen by means of a return conveyer and a crossover chute. Any material still too large to pass the top screen is routed through the  roll crusher or hydrocone  crusher again.  This  is  known  as  a  “closed  circuit”  crushing system. It is obvious that the product setting on the  roll crusher or hydrocone crusher  should be set at or slightly less than the opening size of the top screen. The material retained on the bottom screen is the product that is routed through a chute to a conveyer and then delivered to the product stockpile. Material too small to meet product specifications is screened out by the  bottom  screen  and  delivered  to  a  by-product stockpile. The secondary unit includes a bar type of scalping screen that can be inserted into the top rim of the sloping hopper. When inserted, this screen causes any oversized material to be scalped and rolled off of the screen and to the rear of the machine. The same factors discussed for the jaw crusher also affect the production of the roll crusher. By comparison, dual roll crushers have a limited “stage of reduction” capability.  “Stage  of  reduction”  is  the  difference, expressed  in  inches  or  centimeters,  between  the maximum input and maximum output size of material due to a single crushing action.  A stage of reduction of 3 inches (7.6 cm) indicates a 3-inch (7.6-cm) reduction in maximum particle size. This reduction capability is a function of the diameter of the rolls and of the nature of the roll shell surface; therefore, every different size and combination of roll shells has a somewhat different stage  of  reduction  capacity.  For  this  reason  the maximum size of material to be fed to a dual roll crusher is critical. The maximum allowable feed size is the sum of the stage of reduction capability and the product setting. If the  maximum  feed  size  is  exceeded,  unsatisfactory results likely to occur are as follows: 1.  Retarded  production 2. Excessive roll shell wear 3.  Excessive  long-and-flat  particles These  units  also  have  a  wide  range  of  production capability. The actual rate of production in tons per hour is usually determined by the screening capacity of the screens. In some cases, the rate may be limited by the capacity of the  dual roll crusher  or hydrocone  crusher. Screens Crushed rock particles are separated into two or more particle size ranges by the use of screens (fig. 6-9). Screens are also used to scalp off oversized rock and to Figure  6-9.-Vibrating  screen  box. 6-8







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